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### Transport Processes I: Heat and Mass Transfer

Atoms and electrons Video transcript - Sometimes light seems to act as a wave, and sometimes light seems to act as a particle. And, an example of this, would be the Photoelectric effect, as described by Einstein.

So let's say you had a piece of metal, and we know the nptel heat and mass transfer has electrons. I'm gonna go ahead and draw one electron in here, and this electron is bound to the metal because it's attracted to the positive charges in the nucleus.

If you shine a light on the metal, so the right kind of light with the right kind of frequency, you can actually knock some of those electrons loose, which causes a current of electrons to flow.

## Photoelectric effect (video) | Photons | Khan Academy

So this is kind of like a collision between two particles, if we think about light as being a particle. So Nptel heat and mass transfer gonna draw in a particle of light which we call a photon, so this is massless, and the photon is going to hit this electron, and if the photon has enough energy, it can free the electron, right?

So we can knock it loose, and so let me go ahead and show that.

Nptel heat and mass transfer here, we're showing the electron being knocked loose and so the electron's moving in, let's just say, this direction, with some velocity, v, and if the electron has mass, m, we know that there's a kinetic energy.

The kinetic energy of the electron would be equal to one half mv squared. This freed electron is usually referred to now as a photoelectron.

## Applications of Solar Energy - Google Libros

So one photon creates one photoelectron. So one particle hits another particle.

Nptel heat and mass transfer, if you think about this in terms of classical physics, you could think about energy being conserved. So the energy of the photon, the energy that went in, so let me go ahead and write this here, so the energy of the photon, the energy that went in, what happened to that energy?

### Convective Heat and Mass Transfer NPTEL

Some of that energy was needed to free the electron. So the electron was bound, and some of the energy freed the electron.

I'm gonna call that E naught, the energy that freed the electron, and then the rest nptel heat and mass transfer that energy must have gone into the kinetic energy of the electron, and so we can write here kinetic energy of the photoelectron that was produced.

So, kinetic energy of the photoelectron. So let's say you wanted to solve for the kinetic energy of that photoelectron.

## NPTEL Video Course : Convective Heat and Mass Transfer

So that would be very simple, it would just be kinetic energy would be equal to the energy of the photon, energy of the nptel heat and mass transfer, minus the energy that was necessary to free the electron from the metallic surface.

And this E naught, here I'm calling it E naught, you might see it written differently, a nptel heat and mass transfer symbol, but this is the work function.

Let me go ahead and write work function here, and the work function is different for every kind of metal. So, it's the minimum amount of energy that's necessary to free the electron, and so obviously that's going to be different depending on what metal you're talking about.

All right, let's do a problem.